Surface Science Letters Measurement of Active Carbon on Ruthenium (110): Relevance to Catalytic Methanation

The detection, by Auger electron spectroscopy (AES), of fractional monolayers of carbon on Ru is difficult because of the interference between the Ru (273 eV) and C (272 eV) transitions [I ,2]. In attempting to deal with this problem, two methods of analysis have been used. In the first, the ratio of the 273 eV signal (p-p height) to the 231 eV Ru signal is monitored [3,4]. Because the 231 eV signal is from Ru, unencumbered by C, this ratio grows as carbon accumulates. In the second method, the ratio R* of the positive to the negative portions of the signal at 273 eV is taken [5]. Since the C signal is dominated by a negative feature, while the Ru signal is more nearly symmetrical, this ratio decreases as the surface carbon concentration increases. Neither of these methods is very precise and neither can be reliably used to monitor low levels of carbon. As part of a continuing series of studies of the catalytic methanation reaction (3Hz + CO + CH4 + HaO) [6], we report here the observation and measurement of an AES feature that arises from that form of carbon, customarily called carbidic carbon [6], which is active in the methanation reaction over single crystal Ru(ll0) [7]. This feature can be followed easily at low carbon levels and used for semi-quantitative study of the kinetics of the carbide build-up and removal steps in the methanation reaction. The experiments reported here used a high-pressure reaction chamber linked to an UHV analysis chamber that has been described previously [6]. The Ru(1 IO) sample was cleaned using high temperature oxidation at 10m6 Torr Oa followed by heating in vacuum to 1570 K [8]. In fig. 1, AES spectra in the Ru-C region are shown for (a) a clean surface and (b) a surface exposed to 10 Torr of CO at 550 K for 1000 s. Small amounts of carbon have little influence on the signals at 231 and 273 eV, but in the region between 245 and 265 eV two small peaks appear on the rising portion of the 273 eV signal. The energies of these features (251 and 259 eV) are identical to carbidic carbon features found on Ni [6]. The 259 eV signal may contain contributions